1. Field of the Invention
The present invention relates to a projection exposure apparatus employed in the circuit pattern formation in semiconductor integrated circuits, liquid crystal devices and the like.
2. Related Background Art
The circuit pattern formation in the semiconductor devices or the like requires a step generally called photolithography. Said step generally consists of transferring a pattern of a reticle (mask) onto a substrate such as a semiconductor wafer. The substrate is coated with photosensitive photoresist, onto which the circuit pattern is transferred according to an irradiating light image, namely according to the pattern of transparent areas of the reticle. In a projection exposure apparatus (such as stepper), the image of the circuit pattern formed on the reticle is focused, by projection, onto the substrate (wafer) through a projection optical system.
In an optical system for illuminating the reticle, there is employed an optical integrator such as a fly's eye lens or optical fibers, in order to obtain uniform illuminating light intensity on the reticle. In case the fly's eye lens is employed for optimizing the uniformity of illumination, a relation of Fourrier transformation substantially stands between the focal plane at the reticle side (exit side) and the reticle plane (pattern bearing face), and such relation of Fourrier transformation stands also between the focal plane at the reticle side and the focal plane at the light source side (entrance side). Consequently, the pattern-bearing face of the reticle and the focal plane of the fly's eye lens at the light source side (more precisely the focal planes, at the light source side, of individual lenses of the fly's eye lens) are in an imaging (conjugate) relationship. Consequently, on the reticle, the illuminating lights from the optical elements (secondary light source images) of the fly's eye lens are mutually added (superposed) through a condenser lens or the like, thereby being averaged and providing improved uniformity of the illumination intensity on said reticle.
In the conventional projection exposure apparatus, the light amount distribution of the illuminating light beam, entering the entrance face of the optical integrator, such as the fly's eye lens mentioned above, is made substantially uniform in a substantially circular area (or a rectangular area) around the optical axis of the illuminating optical system.
FIG. 16 schematically shows the configuration of a conventional projection exposure apparatus (stepper) as explained above, wherein an illuminating light beam L140 illuminating a pattern 10 of a reticle 9, through a fly's eye lens 41c, a spatial filter (diaphragm) 5a and a condenser lens 8 in the illuminating optical system. Said spatial filter 5c is positioned at a focal plane 414c of the fly's eye lens 41c at the reticle side, namely a Fourrier transformation plane 17 for the reticle pattern 10 (hereinafter called pupil plane) or in the vicinity thereof, and is provided with a substantially circular aperture with the center at the optical axis AX of a projection optical system 11, in order to limit the secondary light source image (surface light source image), formed on the pupil plane, into a circular area. The illuminating light beam, thus transmitted by the pattern 10 of the reticle 9, is focused by the projection optical system 11 onto a photoresist layer on a wafer 13. In this configuration, the ratio of the numerical aperture of the illuminating optical system (41c, 5a, 8) to that of the projection optical system 11 at the reticle side, or so-called .sigma.-value, is determined by the diaphragm (for example by the aperture of the spatial filter 5a) and is generally selected within a range of 0.3 to 0.6.
The illuminating light beam L140 is diffracted by the pattern 10 formed on the reticle 9, thus generating a 0th-order diffracted light D.sub.0, a 1st-order diffracted light D.sub.p and a -1st-order diffracted light D.sub.m. Said diffracted lights (D.sub.0, D.sub.m, D.sub.p) are condensed by the projection optical system 11, thereby generating interference fringes on the wafer 13. Said interference fringes constitute the image of the pattern 10. In this state, the angle .theta. at the reticle side between the 0th-order diffracted light D.sub.0 and the .+-.1st-order diffracted lights D.sub.p, D.sub.m is given by a relation sin.theta.=.lambda./P wherein .theta. is the wavelength of the exposing light, and P is the pitch of the pattern.
As the pattern pitch becomes finer, sin.theta. becomes larger, and, when it exceeds the numerical aperture NA.sub.R at the reticle side of the projection optical system 11, the .+-.1st-order diffracted lights D.sub.p, D.sub.m are limited by the effective diameter of the pupil (Fourrier transformation plane) 12 in the projection optical system 11 and become unable to pass through said projection optical system. In such state the wafer 13 receives the 0th-order diffracted light D.sub.0 only, and the interference fringes are not formed thereon. Thus, in case of sin.theta.&gt;NA.sub.R, there cannot be obtained the image of the pattern 10, so that the pattern 10 cannot be transferred onto the wafer 13.
Based on these facts, the pitch P satisfying the relation sin.theta.=.lambda./P.congruent.NA.sub.R has been given, in the conventional projection exposure apparatus, by the following equation: EQU P.congruent..lambda./NA.sub.R ( 1)
Since the minimum pattern size is equal to a half of the pitch P, it is about 0.5.times..lambda./NA.sub.R, but, in the practical photolithographic process, there is required a certain depth of focus in order to cope with the curvature of the wafer, topography on the wafer resulting from the process, or the thickness of the photoresist itself. For this reason the minimum resolvable pattern size in the practical process is represented as k..lambda./NA.sub.R, wherein k is called process coefficient and is generally in a range of 0.6 to 0.8. As the ratio of the numerical aperture NA.sub.R at the reticle side to the numerical aperture NA.sub.W at the wafer side is equal to the imaging magnification of the projection optical system, the minimum resolvable pattern size on the reticle is represented by k..lambda./NA.sub.R while the minimum resolvable pattern size on the wafer is represented by k..lambda./NA.sub.W =k..lambda./B.NA.sub.R, wherein B is the imaging magnification (reduction ratio).
Consequently, for transferring a finer pattern, it has been considered necessary to employ the exposure light source of a shorter wavelength or to adopt the projection optical system of a larger numerical aperture. It is naturally conceivable to optimize both the exposure wavelength and the numerical aperture.
However, in the conventional projection exposure apparatus as explained above, the use of an illumination light source of a shorter wavelength is presently considered difficult, for example because of absence of suitable material adapted for use as transmissive optical members. Also the numerical aperture of the projection optical system is already close to the theoretical light, so that a further increase is almost unexpectable. Besides, even if a further increased numerical aperture is achievable, the depth of focus represented by .+-..lambda./2NA.sup.2 rapidly decreases with the increase of the numerical aperture, so that the deficiency in the practically required depth of focus becomes conspicuous.
On the other hand, so-called phase shift reticle, in which the phase of the light transmitted by particular portions of the transparent circuit patterns of the reticle is shifted by .pi. with respect to the transmitted from other portions, is proposed for example in the Japanese Patent Publication No. 62-50811, and such phase shift reticle enables transfer of the patterns finer than in the prior art.
However such phase shift reticle still has various problems, as it is expensive because of the complex manufacturing process, and as the methods for inspection and for repair have not been established.
Therefore, in the projection exposure technology without the phase shift reticle, attempts are being made to improve the resolution of pattern transfer through an improvement in the illuminating method for reticle. One of such illuminating methods consists of providing the spatial filter 5a shown in FIG. 16 with an annular aperture, in order to cut off the illuminating light beam present around the optical axis of the illuminating optical system on the Fourrier transformation plane 17, thereby giving a certain inclination to the illuminating light beam reaching the reticle 9.
However, such special illuminating method, as employing the annular distribution of the illuminating light beam in the Fourrier transformation plane of the illuminating optical system, certainly realizes an improvement in the resolution even with an ordinary reticle, but is associated with a drawback of difficulty of securing uniform illumination intensity over the entire reticle. Also a system provided merely with a member for partially cutting off the illuminating light beam, such as the spatial filter as shown in FIG. 16, naturally results in a significant loss of the illumination intensity on the reticle or on the wafer, thus leading to another problem of increased exposure time, resulting from the lowered illumination efficiency. Furthermore, the Fourrier transformation plane in the illuminating optical system, where the light beam from the light source is concentrated, shows significant temperature increase by the light absorption by the light intercepting member such as the spatial filter, and suitable measure, such as air cooling, has to be considered against the deterioration in performance resulting from thermal variation in the illuminating optical system.